1. Field of the Invention
The present invention relates to a method for guiding an electromagnetic radiation, in particular in an integrated optical device for optical telecommunications.
2. Description of the Related Art
In optical telecommunication systems, information is typically coded in short optical pulses by suitable optical sources, such as light-emitting diodes (LEDs) or semiconductor lasers, which pulses are transmitted along an optical-fibre network and received by photodetectors. Many different signals can be transmitted using a single wavelength of light by interweaving the pulses from different sources, a technique known as time-division multiplexing (TDM).
A simple way of increasing the amount of data that can be transmitted by a single optical fibre is to make the incoming electronic bits as short as possible. Current optical telecommunication systems have achieved data rates up to 40 gigabits per second.
Recently, transmission capacity has been increased by means of dense wavelength division multiplexing (DWDM), which requires a very stable emitting laser and, at the receiver, very narrow linewidth filters and optical switches for separating individual wavelength channels and routing them to the appropriate destinations. Due to the large number of individual components in a DWDM system, integrated optical circuits have been developed. Integrated optical circuits may be either monolithic or hybrid and comprise active and passive components, typically realized on a semiconductor or dielectric substrate, used for coupling between optoelectronic devices and providing signal processing functions.
Improvements in optical integration have recently been obtained by the use of photonic crystals. Photonic crystals are dielectric structures having a periodic variation (or modulation) of the dielectric constant along one, two or three directions of space (and the crystal is therefore referred to as a 1-D, 2-D or 3-D photonic crystal). A 2-D photonic crystal typically comprises a piece of dielectric material (for example an optically thin slab) wherein a periodic array of regions of different refractive index is realized. These regions may be defined, for example, by cylinders filled of air, wherein the refractive index is substantially equal to 1. The regions of lower refractive index can be considered as scattering centres, and light at predetermined wavelengths is coherently scattered.
Motion of photons entering such crystal is, depending on-their energy, influenced in a different way by said refractive index variations; in particular, photons will tend to be confined and trapped or be allowed to propagate according to their wavelength. In particular, it is possible to identify energy bands, wherein light transmission is permitted, and “photonic band gaps”, separating these bands, wherein light transmission is forbidden.
Photonic crystals have been made suitable to guide light by providing waveguiding regions wherein the dielectric material is homogeneous and have therefore no variation of the refractive index. These regions, which are typically considered as regions of “defects”, since they break the periodicity of the structure, are suitable to guide electromagnetic radiation at wavelengths within photonic band-gaps. For example, in a 2-D photonic crystal having a periodic array of holes, the absence of a hole defines a defect and the absence of one or more lines of holes defines a corresponding line of defects.
U.S. Pat. No. 5,999,308, for example, describes an all-optical or optoelectronic integrated circuit wherein a photonic crystal defines a waveguide connecting two devices, suitable to guide electromagnetic radiation at wavelengths within the photonic band-gap. The photonic crystal has a triangular array of rods, for example filled of air, and the waveguide is defined by a line of “defects”, i.e. a linear region wherein rods are absent.
The Applicant observes that, in integrated optical circuits, light guiding has always been performed by means of waveguides of predetermined and unchangeable direction, dimension and position, by taking advantage either of the internal reflection properties of the interface between two materials of different refractive index, as in the more conventional waveguides in integrated optics, or of the light confinement effect of a region of defects in a photonic crystal.
Photonic crystals, whose guiding properties within region of defects have been amply investigated and demonstrated under various conditions and configurations, have also been the subject of studies under conditions of absence of defects, i.e. under condition of regular and continuous periodicity. For the purposes of the present invention, with “photonic crystals having regular periodicity” it is intended a photonic crystal wherein the characteristics of its periodic array do not vary at least in a region thereof of intended light propagation.
The article of P. Etchegoin and R. T. Phillips, “Photon focusing, internal diffraction, and surface states in periodic dielectric structures”, Physical Review B, Volume 53, Number 19, 15 May 1996-1, takes advantage of some analogies between electrons in semiconductors and electromagnetic waves in periodic dielectric structures for providing a method for calculation the band structure of a 2-D periodic dielectric structure. Moreover, this article deals with the phenomenon of photon focusing emitted by a source point in these structure, in analogy with the phenomenon of acoustic phonon focusing, showing what shape shall have the kx–ky diagram of the wave vector k to have focusing of light along predetermined directions.
The Applicant observes that, besides having being studied only at theoretical level, applications of photon focusing would be of limited use in integrated optics.
The article of Marko Lon{hacek over (c)}ar, Jelena Vu{hacek over (c)}kovi{hacek over (c)} and Axel Scherer, “Three-dimensional analysis of dispersion properties of planar photonic crystals”, Proceedings of PECS III conference (June 2001), St Andrew's, Scotland. shows that a planar (i.e. 2-D) photonic crystal may have, under certain conditions, a self-collimation effect in the second energy band (i.e. the energy band over the fundamental band). As disclosed in the article, these conditions determine a negative group velocity.